1. Field of the Invention
The invention relates to the field of high temperature elastomeric materials, specifically, the use of a class of cross-linked organic polymer materials in high temperature end applications as elastomers where traditional and/or high purity elastomers lose performance due to polymer degradation.
2. Description of Related Art
Fluorine-containing elastomers, particularly perfluoroelastomers (FFKM) that include tetrafluoroethylene (TFE) and other fluorinated monomer units are known and employed in end applications where materials are required that exhibit excellent chemical resistance, solvent resistance and heat resistance. They are widely used for sealing and other products intended for use in harsh environments. Further, FFKMs are employed in end applications where a high degree of purity is required in addition to chemical resistance. As technology advances, the characteristics required even for such highly resistant compounds continue to be more rigorous. In the fields of aeronautics, downhole oil drilling, aerospace, semiconductor and chemical and pharmaceutical manufacturing, sealing properties and other elastomeric properties continue to demand the ability to function under ever increasing harsh chemical environments that are also subject to high temperature environments of not less than 300° C. The ability of such materials to withstand high temperature environments has become increasingly important.
While FFKMs provide excellent chemical and plasma resistance, in their unfilled state they typically have weaker mechanical properties. Thus, to achieve satisfactory compression set resistance and mechanical properties it is generally known in the art to include fillers or other reinforcing systems. It is a goal in the art to find ways to blend, modify or fill such materials to make them useful in high temperature end applications and form molded parts that are capable of withstanding deformation and that can withstand ever increasing rigorous conditions. FFKM materials are typically prepared from perfluorinated monomers, including at least one perfluorinated cure site monomer. The monomers are polymerized to form a curable perfluorinated polymer having the cure sites thereon intended for cross-linking upon reaction with a curative or curing agent. Upon curing (cross-linking), the base polymer material becomes elastomeric in nature and exhibits elastomeric characteristics.
Typical fillers used in the semiconductor and other industries to enhance mechanical properties, while trying to avoid diminishing chemical and/or plasma resistance, include carbon black, silica, alumina, TFE-based fluoroplastics, barium sulfate and other polymers and plastics. Sometimes blends of one or more FFKM curable polymers are made to achieve varying properties as well in attempts to improve such materials to meet the challenge of higher thermal, chemical and plasma resistant property requirements for various end applications without sacrificing mechanical and sealing properties.
Use of fluoropolymeric fillers in such compositions can also sometimes contribute negatively to a relatively high compression set particularly in end applications at higher temperatures (e.g., >300° C.). Moldability and bondability can also be limited due to use of such fluoropolymeric fillers.
Various polymers have also been developed with unique cure systems to provide base FFKM compounds that have improved heat characteristics. One example of this is U.S. Pat. No. 6,855,774. The cross-links formed are described as contributing to increased heat resistance. U.S. Pat. No. 6,878,778 further teaches curatives that are described as contributing to resulting end materials having excellent chemical resistance and mechanical strength as well as heat resistance at high temperatures.
Blended FFKMs have also been developed to achieve unique properties. FFKMs such as those formed from U.S. Pat. Nos. 6,855,774 and 6,878,778 and other FFKMs as well have been blended. U.S. Pat. No. 8,367,776 describes compositions of such polymers as well as with one or more additional FFKM(s), wherein two of the FFKM compounds in the composition differ in terms of their perfluoroalkyl vinyl ether (PAVE) monomer content by about 5 to about 25 mole percent. Such blends are described as providing the ability to form compositions which can function well without the use of fluoroplastic fillers and are alternatives to and in some cases improvements over such filled materials. Such blends provide crack-resistance in the presence of harsh chemicals, and good thermal and plasma resistant properties.
U.S. Patent Publication No. 2012/0077935 A1 describes a blend of two or more FFKMs, one of which is a high-TFE content curable perfluoropolymer (as in U.S. Pat. No. 8,367,776) and one of which has a fluoroplastic incorporated in the matrix of a second curable perfluoropolymer. The combined materials provide improved high temperature properties. Such materials are the state of the art in high temperature elastomers for use in demanding environments where chemical and/or plasma resistance are required.
While technology continues to strive to improve FFKM mechanical and compression set performance at high temperatures and in increasingly harsh environments, while retaining the beneficial chemical and/or plasma resistance of these materials due to their level of chemical purity and inertness, there remain performance issues which have become of increasing focus in the art as end users continue to push operating conditions for such materials. As the temperature increases, FFKMs tend to thermally degrade which limits their useful range. While additives and various blending and/or curative modifications attempt to push the useful temperature range higher, there are still use limits.
Other polymers are well known for high temperature use but are not usually employed in all harsh environments when a combination of both good mechanical and elastomeric properties are needed. For example, aromatic polymers such as polyarylenes are known for having thermally stable backbones, but are not generally suitable in end applications where elastomeric behavior is desired. Attempts in the art have been made to use cross-linking of such thermally stable polymers, that are otherwise not elastomeric at room temperature, so as to use the cross-linked materials at a service temperature above their glass transition point (Tg).
WO 2011/071619 A1 discloses use of high temperature sealing elements to avoid degradation in downhole use, which elements incorporate polyetherether ketone (PEEK) having N-Rx-N crosslinking groups linked to the PEEK backbone through C—N bonds.
Similarly, in J. L. Hendrick et al., “Elastomeric Behavior of Cross-linked Poly(aryl ether ketone)s at Elevated Temperatures,” Polymer, Vol. 33, No. 23, pp. 5094-5097 (1992), PEEK is crosslinked by maleic anhydride via oligomer end groups to form a PEEK that exhibits elastomeric properties above its Tg. However, such systems have not yet achieved the high temperature properties and/or hydrolytic stability desired to make them useful as an alternative to FFKMs in high temperature end applications which require the right balance of mechanical and elastomeric properties.
U.S. Patent Publication No. 2013/0012635 A1 discloses thermoplastic materials useful as a shape memory material and articles formed from the thermoplastic materials by heating the shape memory polymer above its Tg, shaping the polymer and then fixing its shape into an article by cooling below the Tg. In use, such shaped articles are heated above their Tg and recover the first molded shape. The polymers suggested for use are those having thermal stability over 200° C. which may be cured in the presence or absence of oxygen. Crosslinkers such as sulfur, silica, quinone, peroxy compounds, metal peroxide, metal oxides and combinations of these cross-linkers can be used with the shape memory polymers for cross-linking.
Some of the prior art systems attempting such high temperature elastomeric end products by cross-linking use complex chemical synthesis to include specific functional groups on or in the polymer. This approach limits the ability to customize cross-link density as the polymer is fixed at the synthesis stage. Greater flexibility would allow for the ability to customize the end materials for different uses.
Lastly, FFKMs are not known as being very strong elastomers, and as noted above, attempts to improve strength include the use of fillers or specialty blends. However, this is tolerated due to their high chemical resistance. Filler systems used to attempt to improve mechanical strength can be a drawback due to thermal stability. However, if thermal stability could be improved and better mechanical properties achieved, a material would be available in the art to meet the ever increasing needs a high temperatures and demanding environments. More products could be designed that are now not possible due to limitations in available materials.
As such, there is a continued need in the art for improved, chemically-resistant materials, with good mechanical properties that can exhibit elastomeric behavior for use in environments that include exposure to harsh chemicals and/or extreme temperature and/or pressure conditions.